1. Field
This disclosure is generally related to an Ethernet Passive Optical Network (EPON). More specifically, this disclosure is related to multiple EPONs sharing a common downstream link.
2. Related Arts
In order to keep pace with increasing Internet traffic, network operators have widely deployed optical fibers and optical transmission equipment, substantially increasing the capacity of backbone networks. A corresponding increase in access network capacity, however, has not matched this increase in backbone network capacity. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks still presents a severe bottleneck in delivering large bandwidth to end users.
Among different competing technologies, passive optical networks (PONs) are one of the best candidates for next-generation access networks. With the large bandwidth of optical fibers, PONs can accommodate broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, PONs can be built with existing protocols, such as Ethernet and ATM, which facilitate interoperability between PONs and other network equipment.
Typically, PONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and the premises of the customers. The “first mile” is generally a logical point-to-multipoint network, where a central office serves a number of customers. For example, a PON can adopt a tree topology, wherein one trunk fiber couples the central office to a passive optical splitter/combiner. Through a number of branch fibers, the passive optical splitter/combiner divides and distributes downstream optical signals to customers and combines upstream optical signals from customers. Note that other topologies, such as ring and mesh topologies, are also possible.
Transmissions within a PON are typically performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which can be an external network belonging to, for example, an Internet service provider (ISP) or a local exchange carrier. The ONU can reside in the residence of the customer and couples to the customer's own home network through a customer-premises equipment (CPE).
FIG. 1A illustrates a passive optical network including an OLT (located at a central office) and a number of ONUs (located at customers' premises) coupled through optical fibers and a passive optical splitter (prior art). A passive optical splitter 108 and optical fibers couple ONUs 102, 104, and 106 to an OLT 100. Although FIG. 1 illustrates a tree topology, a PON can also be based on other topologies, such as a logical ring or a logical bus. Note that, although in this disclosure many examples are based on EPONs, embodiments of the present invention are not limited to EPONs and can be applied to a variety of PONs, such as ATM PONs (APONs) and wavelength domain multiplexing (WDM) PONs.
FIG. 1B presents a block diagram illustrating the layered structure of a conventional EPON (prior art). The left half of FIG. 1B illustrates the layer structure of an Open System Interconnection (OSI) model including an application layer 110, a presentation layer 112, a session layer 114, a transport layer 116, a network layer 118, a data link layer 120, and a physical layer 122. The right half of FIG. 1B illustrates EPON elements residing in data link layer 120 and physical layer 122. EPON elements include a media access control (MAC) layer 128, a MAC control layer 126, a logic link control (LLC) layer 124, a reconciliation sublayer (RS) 130, medium interface 132, and physical layer device (PHY) 134.
In an EPON, communications can include downstream traffic and upstream traffic. In the following description, “downstream” refers to the direction from an OLT to one or more ONUs, and “upstream” refers to the direction from an ONU to the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, data packets are broadcast by the OLT to all ONUs and are selectively extracted by their destination ONUs. Moreover, each ONU is assigned one or more Logical Link Identifiers (LLIDs), and a data packet transmitted by the OLT typically specifies an LLID of the destination ONU. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler to the OLT.
In order to avoid collision of upstream transmissions from different ONUs, ONU transmissions are arbitrated. This arbitration can be achieved by allocating a transmission window (grant) to each ONU. An ONU defers transmission until its grant arrives. A multipoint control protocol (MPCP) located in the MAC control layer can be used to assign transmission time slots to ONUs, and the MPCP in an OLT is responsible for arbitrating upstream transmissions of all ONUs coupled to the same OLT.
Due to the splitting loss at passive optical splitter 108, the number of ONUs coupled to an OLT is limited, thus limiting the number of subscribers within a PON. In order to increase the number of subscribers, the carrier needs to install more OLTs in the central office. Because OLTs are expensive, it is desirable to find an alternative that can allow more subscribers to couple to one OLT.